use s for how much Stephanie spent, and b for how much Brittany spent.
s+b=118
s=2b-8
put the value for s in the first equation
(2b-8)+b=118
then combine like terms
3b-8=118
move the -8 over
3b=126
divide by 3
b=42
now put this into the second equation
s=2(42)-8
now simplify
s=84-8
s=76
now check by putting both values in the first equation
42+76=118
118=118
so, Brittany spent $42 and Stephanie spent $76